Heavy oil is commonly produced by first heating the heavy oil to reduce its viscosity. For example, steam assisted gravity drainage “SAGD” operations are commonly employed to produce heavy oil in tar sands formations. In such SAGD applications, twin horizontal wells having a vertical separation distance typically in the range from about 4 to about 20 meters are drilled. Steam is injected into the upper well to heat the tar sand. The heated heavy oil contained in the tar sand and condensed steam may then be produced from the lower well. The success of such heavy oil recovery techniques is often dependent upon producing precisely positioned twin wells having a predetermined relative spacing in the horizontal injection/production zone (which often extends up to and beyond 1500 meters in length). Positioning the wells either too close or too far apart may severely limit production, or even result in no production.
Active magnetic ranging techniques are commonly utilized SAGD applications. In one known active ranging methodology (e.g., as disclosed in U.S. Pat. No. 5,485,089), a high strength electromagnet is pulled down through a cased target well during drilling of a twin well. An MWD tool deployed in the drill string measures the magnetic field during drilling of the twin well. In another known active ranging method (e.g., as disclosed in U.S. Pat. No. 5,589,775), a magnet is mounted on a rotating sub below a drilling motor (deployed in the twin well). A wireline surveying tool is pulled down through the cased target well and measures the magnitude and direction of the magnetic field during drilling of the twin well. Both methods utilize the magnetic field measurements to compute a range and a bearing (a distance and a direction) from the twin well to the target well and to guide continued drilling of the twin.
The above described active ranging methods, while utilized commercially, are known to include several significant drawbacks. For example, these methods require simultaneous and continuous access to both the twin and target wells. Such continuous, simultaneous access to both wells tends to be labor and equipment intensive (and therefore expensive) and can also present safety concerns. These methods also require precise axial alignment between the magnetic source deployed in one well and the magnetic sensors deployed in the other. Misalignment can result in a misplaced twin well, which can have a significant negative impact on future well productivity. Moreover, the steps taken to assure proper alignment (such as making magnetic field measurements at multiple longitudinal positions in one of the wells) are time consuming (and therefore expensive) and may further be problematic in deep wells. Other drawbacks are described in more detail in U.S. Pat. No. 7,656,161, which is incorporated by reference in its entirety herein.
U.S. Pat. No. 7,812,610 and U.S. Patent Publications 2009/0260879 and 2011/0079431 disclose active magnetic ranging methodologies using an insulative gap and an electric dipole source. The electric dipole generates a magnetic field that may be utilized in the magnetic ranging measurements. Such methods may not always require simultaneous access to both wells. However, they generally require a detailed knowledge of the formation resistivity and structure in order to determine an accurate distance between the two wells.
Another magnetic ranging methodology (referred to herein as enhanced passive ranging) involves imparting a permanent magnetization to the casing string in the target well. The permanent magnetization may be imparted before or after deployment of the casing string in the target well, for example as disclosed in U.S. Pat. Nos. 7,538,650, 7,656,161 and 7,712,519 and U.S. patent application Ser. No. 12/962,058, each of which is incorporated by reference in its entirety herein. An MWD tool deployed in the drill string measures the magnetic field during drilling of the twin well. U.S. Pat. Nos. 7,617,049 and 8,010,290, each of which is incorporated by reference in its entirety herein, disclose various methods for computing a distance and a direction between the twin and target wells.
Enhanced passive ranging techniques have been utilized commercially and advantageously overcome many of the draw backs associated with the active ranging methodologies described above. However, there remains room for improvement and for the development of other magnetic ranging methodologies. For example, magnetizing large numbers of casing tubulars, storing the magnetized tubulars, and deploying the magnetized tubulars in the target well can introduce technical and logistical challenges. In-situ magnetization of the target well casing string can also be challenging and requires temporary access to the target well.